Weed species have long been a problem in cultivated fields. Although once a labor intensive operation, weed control has been made easier by the availability of efficient weed killing chemical herbicides. The wide-spread use of herbicides, along with improved crop varieties and fertilizers, has made a significant contribution to the “green revolution” in agriculture. Particularly useful herbicides are those that have a broad spectrum of herbicidal activity. Unfortunately, broad spectrum herbicides typically have a deleterious effect on crop plants exposed to the herbicide. One way to overcome this problem is to produce plants that are tolerant to certain broad spectrum herbicides.
One example of a broad spectrum herbicide is N-phosphonomethyl-glycine, also known as glyphosate. Glyphosate has been used extensively by farmers worldwide for controlling weeds prior to crop planting, for example, in no-till farming. In addition, glyphosate is an efficient means to control weeds and volunteer plants between production cycles or crop rotations. Glyphosate does not carry-over in soils after use, and it is widely considered to be one of the most environmentally safe and broadly effective chemical herbicides available for use in agriculture.
Glyphosate kills plants by inhibiting the shikimic acid pathway. This pathway leads to the biosynthesis of aromatic compounds, including amino acids, vitamins and plant hormones. Glyphosate blocks the condensation of phosphoenolpyruvic acid (PEP) and 3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid by binding to and inhibiting activity of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase, commonly referred to as “EPSP synthase,” and “EPSPS.”
Unfortunately, no crop plants are known that are naturally tolerant to glyphosate and, therefore, the utility of this herbicide for weed control in cultivated crops had been limited. One method to produce glyphosate tolerant crop plants is to introduce a gene encoding a heterologous glyphosate tolerant form of an EPSPS gene into the crop plant using the techniques of genetic engineering. Using chemical mutagenesis, glyphosate tolerant forms of EPSPS have been produced in bacteria and the heterologous genes were introduced into plants to produce glyphosate tolerant plants (see, e.g., Comai et al., Science 221:370-71 (1983)). The heterologous EPSPS genes are usually overexpressed in the crop plants to obtain the desired level of tolerance.
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